1. Field of the Invention
The present invention relates generally to aircraft, and more specifically to an improved helicopter having coaxial, counter-rotating rotors.
2. Description of the Prior Art
Helicopters generally, and coaxial rotor helicopters in particular, have been known and practiced worldwide for many years. Helicopters have many advantages over fixed wing aircraft, including the ability to take off and land vertically, and to maneuver horizontally in any direction while airborne, including while in a hover at zero or near zero speed. This gives them a high degree of flexibility with regard to landing sites, and the ability to transport passengers and cargo to locations not accessible by fixed wing aircraft.
A disadvantage of helicopters with respect to fixed wing aircraft relates to their relative higher difficulty to fly. Fixed wing pilots seldom become helicopter pilots due to the increased time and expense required to master the additional complexities of helicopter piloting. This means that fixed wing pilots are often unable to take advantage of the helicopter's ability to be flown into areas not accessible to fixed wing aircraft. Furthermore, traditional helicopters are much more difficult to fly under instrument meteorological weather conditions (IMC) than fixed wing aircraft, generally limiting their operation under IMC or requiring costly artificial stabilization.
Traditional single rotor helicopters must utilize a tail rotor or similar thrust device acting around the yaw axis, generally referred to as an anti-torque device, in order to overcome the yawing moment naturally generated by the main rotor. This moment tends to yaw the fuselage in a direction opposite to the direction of main rotor rotation. Forward thrust to overcome drag must be provided by the same main rotor that generates the lift for the aircraft which, when combined with the need to control yaw, results in an aircraft that is more complex to control than a fixed wing aircraft.
The fuselage yaw moment imposed naturally by the main rotor can be eliminated through the use of coaxial, or tandem, counter-rotating main rotors, as known in the art. The natural yaw moment created by a single rotor is essentially cancelled by the second rotor, making yaw control in forward flight a matter of a relatively small power requirement. In conventional coaxial rotor and tandem rotor helicopters, forward thrust to overcome aerodynamic drag is generated by essentially the same mechanism used for a single rotor helicopter; forward thrust is created by tilting the main rotors forward to provide a forward vector component to the rotors' lift vector.
Small, dual coaxial rotor aircraft have been proposed in the art. For examples, patents have been issued to Leon, U.S. Pat. No. 5,370,341; Nolan et al, U.S. Pat. No. 5,791,592, and Norris, U.S. Pat. No. 6,460,802.
In addition, the science and application of coaxial rotor helicopters has been studied for many years. For example, a NASA report, A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research, Colin P. Coleman, NASA Technical Paper 3675 (1997), summarizes the state of the art of these aircraft as of the late 1990's, and references ongoing work and analysis from around the world. In addition, several Russian coaxial rotor designs have been in use for decades, primarily those built by Kamov.
One relatively new design currently available from Sikorsky, known as the X2, uses dual, coaxial rotors coupled with a rear propeller for forward thrust. For yaw control, the X2 utilizes differential collective pitch adjustment between the two rotors, giving a differential torque that provides yaw to the aircraft. Recently issued patents to Sikorsky for coaxial rotor helicopters include U.S. Pat. No. 7,210,651, to Scott, and U.S. Pat. No. 7,229,251, to Bertolotti et al.
However, the coaxial rotor aircraft currently available are still relatively complex for a pilot to fly. It would be desirable to provide an aircraft that combined the advantages of a helicopter with the simplicity of control of a fixed wing aircraft.